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A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

Sometimes I like to write about things in the sky that I've been studying. Sometimes I like to write about scientific discoveries in the outer solar system. Sometimes I even write about wild speculations I have about the solar system. But, every once in a while, I get to just sit back and watch the sky go by.

I love comets. When I first started graduate school to get my Ph.D. in astronomy, I wanted to study the most distant galaxies in the world. But my Ph.D. advisor really wanted me to start by doing a project studying a comet (actually, he wanted all of his graduate students to start with comets, because no one stuck with them; they jumped to galaxies as fast as they could). I fell in love with comets. Mostly, I think, I fell in love with the fact that you could use huge telescope to study things in the sky that you could actually see with your eyes or with binocular or with a camera. Things that were real.

So I was pretty excited about the prospect of Comet Panstarrs close to the tiny tiny crescent moon tonight. We have a great western horizon from my house and I was pretty sure we would have good views. Scientifically, I have nothing at stake. I'm not involved in any attempts to look at the comet with telescopes big or small, on the ground or in space. I just wanted to see it.

So I waited.

The tiny crescent moon was going to be easier to see, so up and down, back and forth, with binoculars I searched. THERE! It was, 25 minutes after sunset, higher than I thought. This was good news. It would be a good ~30 minutes before the comet set. Long enough that even my daughter Lilah would be able to see it.

(Lilah uses a placemat every day that has astronomy pictures [including, yes, Planet Pluto. It was a present. Really] on it, including comets. She is really really excited about seeing one in real life).

I had set out the camera and tripod earlier, and started taking long exposures, hoping to capture the comet. I kept seeing something. Maybe. To the left. Where I knew it should. Be. But? Well? I dunno.

Until, finally, jackpot:

See it? Barely? Something like 6 lunar diameters to the left of the moon?

The first thing that you notice when you look at a spectrum
of Europa -- from the Earth, from a spacecraft, it doesn’t really matter – is
the ice. Ice is everywhere. The spectrum of ice is a very distinctive looking
thing, with a quickly recognizable pattern of regions where the sunlight
reflects strongly from the surface and regions where there is less reflectance
(and remember the regions here means spectral regions, which means,
essentially, we stare at one small spot on the surface, put the light through a
prism to spread it all out, and see which colors of the rainbow are present and
which are absent. In our case our rainbow is in infrared light that your eye
can’t see, but the idea is still the same).

One of the biggest problems with trying to figure out what is
on the surface of Europa was that the spectrograph on the Galileo spacecraft
didn’t have a very fine view of the reflected light coming off the surface. The
analogy I used in Part 1 was that Galileo was looking at fingerprints where you
could only discern the rough pattern and not the individual ridges. You
couldn’t use those fingerprints to know for sure who had smudged your crystal,
though you might be able to rule out some people and you might become more
suspicious of others.

There are two main reasons that the views from Galileo were
not as fine as we would like. First Galileo was old when it arrived at Jupiter.
Serious work began on the spacecraft in 1977, and with typical delays and
atypical space shuttle accidents, it was finally launched, via a space shuttle,
in 1989. Even the trip to Jupiter took longer than initially planned -- the
shuttle accident spawned new rules which required the use of a less powerful
rocket to launch Galileo from the orbiting shuttle -- so Galileo could not go
directly to Jupiter but instead had to get gravity sligshots off of Venus and
Earth before finally heading towards Jupiter and arriving in 1995, nearly
twenty years after construction began. It was old on the first day it took data
at Jupiter. (It was intentionally crashed into Jupiter in 2002 to prevent,
among other things, an accidental crash into Europa, which would clearly
disturb the whales). Not surprisingly,
the old technology was not as good as current technology in seeing precise
spectral fingerprints.

Ever wonder what it would taste like if you could lick the
icy surface of Jupiter’s Europa? The answer may be that it would taste a lot
like that last mouthful of water that you accidentally drank when you were
swimming at the beach on your last vacation. Just don’t take too long of a
taste. At nearly 300 degrees (F) below zero your tongue will stick fast.

The composition of the surface of Europa has been hotly
debated since the Galileo mission attempted to make detailed measurements more
than a decade ago. Galileo’s tool for measuring the composition was
spectroscopy – looking at the sunlight that reflects off of the surface of
Europa and seeing which molecules leave characteristic fingerprints in that
reflected sunlight. It’s a powerful technique, one that led to the initial
discovery of water ice on the Galilean satellite, the discovery of frozen
methane on distant bodies like Eris and Pluto and Makemake, and is even used on
the Earth to map out mineral deposits for potential exploitation.